Conventionally, this type of float valve is disclosed in Japanese Unexamined Patent Publication No. 5-185850 and Japanese Utility Model Publication No. 4-39061, and the structure of the float valve is shown in FIG. 1.
The float valve V0 includes: a cylindrical case 1 having a bottom; float 4 accommodated in the case 1; valve 5 disposed at an upper portion of the float 4, being capable of closing an air efflux channel 7; and compression coil spring 6 disposed between the float 4 and a bottom wall 2 of the case 1. In this structure, the case 1 is fixed onto an upper wall 9 of the fuel tank 8.
On the bottom and side walls 2, 3 of the case 1, there are provided through-holes 2a, 3a for allowing fuel F and air A to flow into the case 1.
The coil spring 6 is used for supporting the float 4 so that the air efflux channel 7 can be closed by the float when a vehicle is inclined or overturned. For example, when the vehicle is overturned, the air efflux channel 7 is placed below the float 4. In this case, a spring force of the spring 6 is determined so that the total force of a mass of the float 4 and a pushing force of the spring 6 can be higher than a buoyancy of the float 4. Of course, the spring force of the spring 6 is determined in the following manner: Before the level of fuel F is raised under the condition that the vehicle is in an appropriate posture, the float 4 is not lifted up by the spring 6 to close the air efflux channel 7, but the total force of the buoyancy of the float 4 and the pushing force of the spring 6 becomes higher than the mass of the float 4 when the float 4 is dipped in fuel 4.
Due to the foregoing, in this type float valve V0, when the level of fuel F in the fuel tank 8 is raised, fuel F flows into the case 1 through the through-holes 2a, 3a, so that the float 4 is raised. Accordingly, when valve 5 closes the air efflux channel 7 fuel F will be prevented from flowing to the outside of tank 8. In this connection, after air has been vented through the air efflux channel 7, it is sent to a canister not shown in the drawing.
However, when fuel F is fed to the fuel tank 8 during refueling, a large amount of fuel F is fed in a short period of time. Therefore, the following problems may be encountered in the conventional float valve V0.
In the case of refueling, air A may suddenly flow into the case 1 through the through-holes 2a, 3a, so that air A flows into case 1 and out 8 the air efflux channel 7 at high speed. Accordingly, such flow may raise the float 4. Under the above condition, fuel F flows into the case 1 through the throughhole 2a on the bottom wall 2 of the case 1, and the float 4 is raised by the buoyancy of fuel F. Then, before the level of fuel F is reached to a position LF which corresponds to a fuel level at which the float 4 is disposed at an appropriate valve closing position, the valve 5 closes the air efflux channel 7 by the high speed air flow action. This valve closing motion tends to occur when buoyancy is activated on the float 4 by fuel F because a downward force of the float 4 is reduced at this time. In this case, the downward force of the float 4 is a value obtained when the buoyancy of the float 4 and the pushing force of the spring are subtracted from the mass of the float 4.
Due to the above valve closing motion, air to be discharged outside the fuel tank 8 is stored in the fuel tank. Therefore, the fuel level in the fuel tank 8 on the fuel feeding side is raised, so that an automatic stopping mechanism of the fuel nozzle, by which fuel is fed into the fuel tank 8, is operated and the fuel feeding operation is stopped. In this way, problems are caused during refueling.